JOURNAL O• GEOP,HY$ICAL RESEARCH, VDL. 94, NO. D3, PAGES 3473-3486, •h•RCH 20, 1989 NO 2 OBSERVATIONS AT 45øS DURING THE DECREASING PHASE OF SOLAR CYCLE 21, FROM 1980 TO 1987 P. V. Johnston and R. L. McKenzie Physics andEngineering Laboratory, Lauder, Department of Scientific andIndustrial Research Central Otago,New Zealand Abstract. Monthly averaged NO 2 columns obtained at twilightat 45øS over the period December 1980 to August 1987are presented. Improvements to the retrieval technique used previously are discussed. The slant columns presented typically range from 3 to 12 x 1016 cm -2 which, because of the geometry at twilight, are approximately 20 timesgreaterthan the vertical columns. In addition to the expected diurnal and seasonal variations, the data also showvariationson longer time scales. The identification of any long-term trends is masked by the effects of the E1Chich6n eruption, which wasprobablyresponsiblefor a decrease in the columnof 1.5 x 1016 cm -2 in 1983. After allowing forthis, there remains a small but significant long-term trend.This trend maybe due to a cooling in the stratosphere that hastaken place over the observation period. Alternatively, there may be a correlation with solar activity. The observation period is too short to be able to establish a definite link, butthe trend is consistent witha decrease of 0.5x 1016 cm -2 in the NO 2 column from solar maximum to solar minimum. The self-consistency from year to year does not support the theory that the "Antarctic ozone hole" is caused by variationsin solar activitywhich changethe production of the nitrogen oxides responsible for the catalytic destruction of ozone. Both the E1 Chich6n eruption and the long-term trend discussed earlier produceadditive rather than multiplicative effects,with similar column reductions occurringin the summer and winter as well as in the morning and evening measurements. 1. Introduction In this paper wepresent monthly averaged NO 2 slant column measurements at 45øS, for the periodDecember 1980to August1987.Long-term changes are emphasized by plotting the results as departures from monthly means. The effects of E1 Chich6n are identified, and we discuss whether the observed changesare real or are due to changes in the zenith scattering geometrycaused by the increasedaerosol. The data is examined for trends which, Copyright 1989 by the American Geophysical Union. Papernumber 88JD03858. 0148-0227/89/88JD-03858505.00 according to Callis and Natarajan [1986a, 1986b], hereafterreferred to as C&N, may be related to changes in solaractivity. C&N postulate that substantial increases in odd nitrogen levels at middle to high southern latitudes, from 1978 to 1982-1984are the principal cause of the "Antarctic ozone hole." 2. Measurement Technique Stratospheric NO 2 columns are measured routinely at sunrise and sunset usinga ground-based spectrometer. The technique usesthe highly structured absorptions by NO 2 whichare present in sunlight (near 450 nm) scattered from the zenith sky at twilight. It was first reportedby Brewer et al. [1973, 1974]and has since been applied by several groups to study extensively the spatial and temporal variability of stratospheric NO 2 [Noxon, 1975, 1979, 1980; Noxon et al., 1979a, 1979b, 1983, Harrison 1979, McMahon and Simmons 1980, McMahon 1984, Syed and Harrison 1980, 1981a,b; McKenzie and Johnston, 1982, 1983;Ridley et al., 1984;Solomon et al. 1987]. Morning and evening twilight spectra in the wavelength range 434.5-449.5nm are obtained as the solar zenith angle (SZA) varies over 90.0___0.5 ø. To remove the Fraunhofer absorption lines present in sunlight, measurements are occasionally made near middayon clear days. The twilight measurement spectra are ratioedwith a selected midday "control" spectrum to remove the large ("'20%) Fraunhoferfeatures. The logarithmof this ratio (twilight over control) spectrum contains mainly NO 2 (<3%) and 0 3 (<1%) absorptions [McKenzie andJohnston, 1982]. On April 18, 1984 the slit width of the spectrometer was doubled, changing the resolution from 0.45 to 0.90 nm. The reduction in photonnoise(factor "' 2) is only slightly offset by a reduction (factor0.89) in the standard deviations of the NO 2 differential cross section (dueto smoothing at lowerresolution). 3. Retrieval ofNO 2 Column Amounts Since the measurements reportedby McKenzie and Johnston [1982] the data reductiontechnique has been improved and the measurementnoise reduced. The control spectra are aligned in wavelength with a reference Fraunhofer spectrum[Delbouille, 1973] to correct for 3473